Equipment manufacturers engaged in the production of flow switches for fluid or gas sensing have various technologies available to them in the design of sensing elements to accommodate a variety of flow rate sensing hardware. Most customers seeking a simple solution to threshold sensing specify sensors to communicate that flow has risen above, or fallen below, a specific setpoint in an application. Typically, for these types of applications, the need is for a switch that is capable of sensing flow in only one direction. Routinely the need is served well by simple switch designs available in a variety of sizes and configurations.
When there is a need to sense flow in both forward and reverse directions in a process, the choices become limited, and manufactures may opt to serve the need by choosing a more complicated, continuous sensing device. Many of these types of sensors have the ability of sensing bidirectional flow by employing the use of electronic circuitry that is capable of processing signals from mechanical or electronic components in the flow stream. Unfortunately, although these technologies may be effective, they are typically more expensive due to their complex nature. In addition, they can be more susceptible to the effects of unintended external electrical interference than the simpler, electronic solution, typically found in a one directional flow switch.
In certain bidirectional flow switches, sometimes referred to as paddle switches, a generally planar paddle extends into a flow path, where the plane of the paddle is oriented perpendicularly to the flow path's general flow direction under zero flow conditions. The planar paddle is hinged at an end so that flow through the flow path impinges upon the paddle and causes it to pivot about the hinge's axis. As the paddle pivots about the hinge from its perpendicular, zero-flow position, a mechanical linkage from the paddle pulls a magnet, which is spring biased away from a reed switch, towards the reed switch. A threshold flow causes the magnet to travel to a position proximate the reed switch so that the reed switch changes from an open state to a closed state. Alternatively, continuous sensing devices, such as, for example, paddle wheel meters or ultrasonic meters, may be more complex and expensive than simple one-direction flow switches. In a paddle wheel switch, a cylindrical wheel has flanges and magnets disposed in a spaced apart manner about its cylindrical outer surface. A portion of this surface extends into the flow path so that fluid flow engages the paddles and correspondingly turns the wheel. The remaining portion of the wheel extends outside the flow path, at which magnetically-sensitive electronics detect movement of the rotating magnets. Such arrangements can be susceptible to damage in response to high flow rates and can require relatively sophisticated circuitry to respond to frequency of magnet movement.